Applicant is aware of at least one attempt in the prior art to address the problem of chip breakage and crushing during the operation of a wing knife frustro-conical chipping head. In particular, applicant is aware of U.S. Pat. No. 5,505,239 which issued Apr. 9, 1996 to Sparks for a Blade Arrangement and Blade Holder for Chipper.
Sparks describes that a common use for such a chipper is to open the faces of a log. He describes the chipping operation formed on a log as removing the log material at two opposed sides, that is, outside of a vertical chord through each side of the log, thereby forming the so-called cant. Sparks identifies that the material removed by the chipper is valuable for making pulp especially if the material is cut into chips of generally consistent size. Sparks identifies that one problem in the prior art in the formation of chips has been that the chips formed by the blade on the chipping head flow past the blade and blade holder to strike an outer support leg supporting the blade holding structure. Sparks identifies as a second problem that in prior art knife formations, where two knives are abutted to form an angle comer or bend, that wood strands are driven into the interface between the two adjacent blades becoming wedged between the blades causing scoring of the opened faces on the cant.
The Sparks device replaces the prior art spaced leg supports with a single support wall that extends from a forward face of the chipping head, referred to as the inner side of the chipping head or the disc side, outwardly and rearwardly in a sweeping concave-like configuration that extends past the opening previously found between the blade holder and the leg supporting the blade holder. Sparks teaches that rearwardly curved or scoop-shaped surface formed in the support wall provides a guide way that receives and directs or guides the chips past the holder in the direction of natural flow of the chips as perceived by Sparks, Sparks stating that it had previously been perceived that the chip flow path flowed around the blade holder and through the opening between the blade holder and the leg support. As the teaching of Sparks would have it, the chips rather than passing through such an opening and under the blade holder, travel in a more direct outward direction so that a large portion impacts against the support leg causing the chips to break up or be crushed thereby rendering them less valuable for pulp production.
The stated objective of Sparks is to simulate the natural direction of chip flow in the radially outwardly diverging scoop formed underneath the blade holder along the supporting wall of the blade holder so as to avoid the direct impact of the chips against the leg support structure of the prior art. The prior art referred to by Sparks is illustrated herein in FIG. 1 for comparison purposes. The structure taught by Sparks is illustrated herein and identified as prior art in FIG. 2.
The present invention is directed to an observation by applicants that perhaps the teachings of Sparks were not entirely complete. In particular, applicants have noticed that wear patterns in the base structure of wing knife holders on Key-knife.TM. and Furano.TM. chipping heads indicate that a considerable volume of chip flow is at least initially directed downwardly. That is, the chip flow is directed radially inwardly relative to the axis of rotation of the chipping head, in a direction substantially perpendicular to the perceived lines of chip flow (indicated as lines 38' and 40') illustrated in the prior art as seen in FIGS. 1 and 2, so as to impinge the base structure. In particular, the primary wear pattern is located at a position on the base corresponding to the first two bolt positions nearest the inner side of the chipping head. For ease of reference the position of the primary wear pattern is indicated in FIG. 2 as 44'.
Applicant believes that the largest percentage of the chip formation occurs at the radially inner portion of the wing knife blade. This may be intuitively substantiated by a review of FIG. 3 and FIG. 3a which diagrammatically illustrate in plan view a log having an opposed pair of planar faces opened by an opposed facing pair of frustro-conical chipping heads so as to produce a cant flowing in the outfeed direction. As may be seen, the radially outermost portions of the wing knife blades engage the outermost edges of the log, that is, they do not have to slice through relatively long cord lengths through the log. Thus the relative volume of wood removed by the outer portions of the wing knife blades is lower, and hence the chip volume and flow rate is lower, than that created by the radially inner portions of the wing knife blades which slice through relatively longer cord lengths as the sides of the log are chipped away until the final cant dimensions are obtained and the cant passes between the opposed facing chipping head hubs.
Notwithstanding the distribution of the relative rate of chip formation along the length of the wing knife blades, the observed wear patterns on the base of prior art wing knife holders indicate significant chip flow against the innermost portion of the knife holder base, that is, adjacent the radially innermnost end of the wing knife blade. It is applicant's belief that such an impinging flow directed against a structure that in the prior art, including the teaching of Sparks, is a substantially flat plate, cannot but increase the incident of chip breakage and crushing. Thus, although no doubt chip breakage did occur in the prior art against the wing knife holder leg support as identified by Sparks, applicant believes that this is secondary damage compared to primary chip damage occurring at the leading edge side of the innermost portion the knife holder base.
Consequently, it is one of the objects of the present invention to provide a wing knife holder structure which is cantilevered in the form of a wing which extends radially outwardly beyond the base of the knife holder so as to minimize obstruction of the chip flow path, and which, contrary to the teaching of Sparks, in the present invention smoothly deflects chip flow initially in a radial inwardly direction while urging the chip flow to the outer side of the chipping head clear of interference from the rotating chipping head structure.